The invention relates to a vessel comprising a chain hawse and an anchor chain of interconnected chain links, the anchor chain running through the chain hawse which ends in an exit opening in the hull of the vessel and having a predetermined radius of curvature near the exit opening.
In known configurations, the chain hawse is curved near the opening in the hull to accommodate different angles of pull on the anchor chain. The chain links in such a case are subject to interlink wear due to constant small movements of the chain links. Also the wall of the chain hawse will in the curved region be subject to wear as the chain links rub against the chain hawse wall. By the curvature of the chain hawse, the interlink wear can be spread over a few chain links.
Most state of the art designs for chain hawses, such as for instance described in OTC publication no. 3855, presented at the 12th Annual OTC in Houston, Tex., May 5-8, 1980. Title: xe2x80x9cStrength of chain tensioned over a curved surfacexe2x80x9d, by John F. Flory, and Steve P. Woehleke, Exxon Research and Engineering Co. are laid out to be able to withstand extreme forces during gale conditions and during high seas when the anchor chain has a maximum angle with respect to the vessel. The curved part of the known chain hawse is long enough to distribute the acting forces over a sufficiently large number of links, such that the bending fatigue of each link is reduced.
From FR-A-260 1322 it is known that for reduction of wear of the chain hawse wall, a steel support element may be applied, having a central groove for the transverse chain links, and forming a support surface for the chain links that are oriented substantially parallel to the chain hawse wall. Such a construction effectively protects the chain hawse wall and stabilises the chain position, but may still result in significant bending forces on each link.
In again another attempt to reduce chain hawse wear, a rub casting is applied on the chain, in combination with a straight or curved chain hawse. The rub casting, which may be formed of steel, prevents the chain links from contacting the chain hawse wall, but causes interlink wear of the one or two links closest to the rub casting.
In all of the above constructions, the interlink wear is relatively large. Furthermore, in the construction without the protective support element and in the construction in which a rub casting is used, chain hawse wear may be problematic. Finally, the use of a curved chain hawse may lead to chain bending fatigue problems due to point contact of the chain with the sides of the hawse on the long sides of the links.
It is an object of the present invention to provide a chain hawse for a dynamically moving anchor chain in which the effects of interlink wear are at least substantially reduced. It is a further object of the present invention to provide a chain hawse wherein the bending fatigue for each chain link as well as chain hawse wear are at least substantially reduced.
Thereto a vessel, according to a first embodiment of the invention, is characterised in that the curvature of the chainhawse is such that the interlink angle is less than 15xc2x0, preferably less than 11xc2x0.
By reducing the radius of curvature of the chain hawse wall, or by using a relatively large number of smaller chain links the interlink wear can be significantly reduced.
The invention is based on the insight that interlink wear and bending fatigue are largely due (e.g. for 90%) to the continuous little dynamic moments of the anchor chain and for less due to extreme weather conditions (e.g. 10%). During normal operation, the chain angle with respect to the horizontal, which normally can amount to for instance about 50xc2x0, varies by a maximum of plus or minus 10 degrees. Contrary to the state of the art chain hawses in which a smaller curvature is distributed over the entire hawse pipe resulting in interlink angles between 20xc2x0 and 70xc2x0, the present invention minimises the interlink wear by the use of relatively small interlink angles in the area of the hawse pipe where most of the normal wear occurs.
According to a further embodiment of the present invention, the curved part of the chain hawse is along at least a part of its length provided with a chain support element for supporting the chain links. The chain support element comprises two oppositely located, parallel ridges extending in the length direction of the chain. Each ridge, when seen along the transverse center line of the chain links that are supported on said ridge, is substantially located within the area that is defined by the internal dimension of the chain links. The internal dimension is defined by the distance between the centers of the parallel chain link parts in a chain link along the transverse centre line.
Hereby, the chain links are supported by the ridges in four points on the short sides of the links. No substantial contact forces along the long sides of the links with the support element are present, such that long side bending and the associated bending fatigue is significantly reduced or eliminated altogether.
Preferably the height of the support ridges is sufficient to maintain a clearance of the chain links with the chain hawse wall for further reduction of long side bending. The supporting ridges may be formed by a stepped support member attached to the chain hawse wall or by the sharp edges of a slanting support member.
According to a preferred embodiment of the present invention, the chain support element comprises two support surfaces that include a predetermined angle, the planes of two adjacent chain links being located on different support surfaces.
By placing the plane of the chain links at an angle with respect to the chain hawse wall and by supporting each link in its plane on the support element, a favourable force distribution is obtained and long side bending is eliminated to a large extent. Preferably the angle between the planes of two adjacent chain links is about 90xc2x0, the angle between a chain link and the chain hawse wall being about 45xc2x0, and the angle between the slanting support surfaces being about 90xc2x0.
In both embodiments wherein support surfaces are used on the chain hawse wall, the interlink angle may be less than 15xc2x0, preferably not more than 11xc2x0.